Wavelength-sensitive photo-sensing (often referred to as color sensing) has applications in a wide range of fields such as medicine and biology, the food, printing, and cosmetics industries, and the like. For example, in the study of cells and tissues, it may be necessary to monitor or detect the transmission and absorption of light of a certain bandwidth by cells and tissues under study.
Semiconductor-based color sensors are known. These sensors typically operate based on the differential absorption of visible light in a solid material such as silicon (Si) based on wavelength. That is, longer wavelength light can penetrate deeper below the surface than shorter wavelength light. Secondly, light is absorbed in photoelectric processes as it interacts with, and loses energy to, electrons in its path. In a semiconductor, when a non-conductive electron obtains sufficient energy, it is excited to the conduction band. This transition generates an electron in the conduction band and a hole in the valence band, both of which can be free-carriers. Thus, a beam of light incident on a semiconductor can generate free-carriers at different depths in the semiconductor depending on its wavelength. Advantageously, free-carriers generated in a depleted region, developed around a reverse biased pn-junction, can be detected by sensing a current from the depleted region. The sensed current can thus indicate the intensity of light absorbed in the depleted region. Therefore, depleted regions formed at different depths in a semiconductor can be used to sense different spectral components of the incident light.
As an example, the spectrum of visible light can be typically resolved into three components: blue, green, and red, which penetrate increasingly deeper into a semiconductor. To detect these three spectral components, the depth of a depletion region of a pn junction may be varied by adjusting the pn junction reverse bias voltage, thus obtaining measurements for the three different components. However, this technique has a disadvantage—it cannot detect different components simultaneously. Alternatively, multiple pn-junctions, may be vertically stacked, to create multiple depleted regions at different depths, so that multiple spectral components can be simultaneously measured.
The known vertically-stacked-junctions, however, also suffer certain shortcomings. For example, for each spectral component, a separate pn-junction is required. To detect three components of light, three pn-junctions are required. This requirement limits the minimum size of each sensing unit and thus the spatial resolution of the sensor.
Accordingly, there is a need for improved methods and devices for sensing color.